The present invention relates to improvements in the dynamic mechanical analysis of thermoplastic material.
Dynamic mechanical thermal analysis (sometimes referred to herein as "DMA") is a technique for measuring mechanical and rheological properties of thermoplastic materials in the solid state as a function of temperature. The technique involves subjecting a specimen, which typically is bar shaped, to bending or flexing at a given temperature and measuring the stiffness or resistance of the material to the imposed deformation. A series of such measurements over a range of temperatures provides information about the thermal behavior and mechanical properties of the material. Glass transitions and melting points are easily detected at temperatures where samples show softening and loss of stiffness.
An existing method for dynamic mechanical analysis utilizes molded bars, available for example in the form of Izod bars, for which little or no sample preparation is required. The thickness of Izod bars is, however, sufficient to create heat transfer limitations which significantly affect the speed of testing. As sample thickness increases, more time must be allowed for thermal equilibrium to be reached. The use of thinner samples or films is a partial solution to the heat transfer problem. However sample preparation becomes labor intensive and extremely time consuming since the sample must be machined to an exact thickness. Uniform thickness is often difficult to maintain and properties may be altered through the process of cutting and machining. Molded bars or parts which require minimal cutting remain the best choice for guaranteeing uniform thickness and unaltered properties.
The accuracy of DMA measurements depends largely on the attainment of thermal equilibrium at each measurement temperature. Some dynamic mechanical analyzers utilize a radiative oven similar to that in a conventional oven. Thermal energy is provided to the sample with heating elements that line the interior of the sample heating chamber. Heat transfer however with a relatively thick sample is poor for conditions under which temperature is continuously changing while measurements are made. There is little or no direct heat transfer in such units. Gas from a liquid nitrogen Dewar is circulated through the sample chamber. However, the gas normally enters the chamber outside an insulation barrier and very little convection directly onto the sample is created. At high temperatures nitrogen flow diminishes and heat transfer becomes purely radiative.
Other dynamic mechanical analyzers, on the other hand, employ forced convection, with heated or cooled gas blown directly into the sample chamber by way of for example a heat gun. Heat transfer is very efficient and thermal equilibrium is approached under most testing conditions. However, sufficient time is necessary for the entire sample to reach a uniform temperature before a measurement is made. This can be achieved using a step method whereby the temperature is increased incrementally and a specified equilibration time is allowed to elapse before obtaining each measurement. Drawbacks of this method are the long time required to complete a series of temperature steps and the limited accuracy which results from the use of temperature increments. Therefore uncertainties in transition temperatures exist. Total measurement time and accuracy of transitions are opposing factors with the step method. Smaller temperature increments improve accuracy but increase measurement time.
Suitable conditions for the step method can be implemented on certain commercially available devices. However, the typical testing time of six hours or more is impractical for large numbers of analyses. The efficient alternative to the step method is a temperature ramp, using a continuous, for example linear increase of temperature at a given rate.
Efficiency of analysis in the temperature ramp mode is much better than that of the step method if a fast heating rate is used. However, heat transfer becomes a severe limitation since the temperature of the sample lags behind the temperature of the surrounding gas. With an increasing temperature ramp, therefore, measured transitions are always different from the true values. Thermal lag within the sample can be minimized if effective heat transfer is provided and a reasonably slow ramp speed is employed.
When a temperature ramp is used under conditions of radiative heat transfer, significant thermal lag between the sample and oven temperatures is observed. The sample is actually being "baked" with the temperature of the interior always cooler than that of the surface. The most direct way to eliminate the thermal gradients within the sample is to use a step method instead of a temperature ramp. However, a significant drawback in this regard is that analysis times are undesirably long for the step method.
Another approach involves characterizing thermal lag in the temperature ramp mode for a given ramp speed and correcting the data by way of computer controls. Corrections can be made to the data using one or more points, however an assumption has to be made that the corrections for thermal lag are material independent, which may not be true if thermal conductivities vary.
Computer software provides the capability to implement corrections for thermal lag which occurs in a temperature ramp mode using devices which operate with radiative heat transfer The corrections can be made using either a single point, which results in simple subtraction of a specified value, or two points, which provides a linear variation of the correction over the entire temperature range. Experience in analyzing low and high temperature transitions on such instruments has shown that thermal lag varies significantly with temperature and therefore a two point correction is likely to be most effective. However, a linear relationship (independent of thermoplastic resin type and composition) is required for accurate and efficient implementation of the computer correction. Multiple corrections that depend on thermoplastic resin type are a possibility, but their utility becomes questionable for complex formulations.
Thermal lag in a temperature ramp mode was found to be very large at low temperatures but tended to decrease at high temperatures. There was not however a linear correlation of lag with temperature. While computer software correction of measured transition temperatures is better than no correction at all, the variability of thermal lag from one thermoplastic to another is a severe limitation.
Accordingly, there exists a need to improve the operation of DMAs which utilize radiative heat transfer, to provide for a more effective heat transfer to the material being tested.